Chromate Ester Research Articles

Application of UV radiation for in-situ Cr(VI) reduction from contaminated soil with electrokinetic remediation

Restoring hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) from contaminated soil is a cost-effective alternative for attenuating Cr(VI) toxicity to the ecosystem. A new electrokinetic remediation (EKR) system with UV light was explored to overcome an energy barrier to catalyze Cr(VI) reduction from the surface soil near the anodic reservoir. Natural organic matters and minerals from the contaminated soil acted as electron donors and catalysts for Cr(VI) photo-reduction and no additional chemical reagent. There was almost no residual Cr(VI) in anolyte after UV/EKR compared with the conventional EKR. The reduction improved the efficiency of EKR in the soil near the anodic reservoir by dropped the Cr(VI) negative mass flux caused by electroosmosis advection and concentration diffusion. The pathways of Cr(VI) photo-reduction are possibly dominated by ligand-to-metal charge transfer, i.e., photocatalytic cyclic reduction by Fe(III)/Fe(II) complexes on the surface of the minerals and in soil pore fluid and the photo-induced decomposition of chromate ester. It is concluded that UV/EKR is a clean, efficient, and low-cost method for remediation of Cr(VI)-contaminated soil.

OXIDATION OF DIOLS BY MORPHOLINIUM FLUOROCHROMATE: A KINETIC AND MECHANISTIC STUDY

The kinetics of oxidation of diols by morpholinium fluorochromate (MFC) has been studied in dimethyl sulfoxide (DMSO). Corresponding hydroxy carbonyl compounds are the main product of oxidation. The reaction is first order in MFC and follows Michaelis-Menten type of kinetics with respect to diols. The reaction is catalyzed by H+ ions. The hydrogen ion dependence has the form: kobs =a+ b[H+]. Oxidation of deuterated ethylene glycol showed substantial primary kinetic isotopic effect (kH / kD = 5.750 at 298 K). Thermodynamic parameters like enthalpy, entropy and free energy of activation are discussed and validity of the isokinetic relationships also established. We have used nineteen different organic solvents for carrying out our analysis. Taft and Swain’s multiparametric equations have been used for the solvent effect analysis. In our experiments rate constants have showed excellent correlation with Taft's Σσ* values along with negative values for the reaction constant ρ*. A suitable mechanistic pathway for the chromate ester formation has also been suggested.

CORRELATION ANALYSES IN THE OXIDATION OF ALIPHATIC SECONDARY ALCOHOLS BY QUINOLINIUM DICHROMATE

Some of the aliphatic secondary alcohols when subjected to oxidation by Quinolinium dichromate (QDC) in dimethyl-sulfoxide (DMSO) as a medium, leading to form a product as analogous ketones. The reaction is found to be in the order of unity in QDC. A Michaelis-Menten type of kinetics, i.e. fractional-order kinetics is observed concerning the secondary alcohols. All the reactions have shown the effect of hydrogen ions. The oxidation of deuterated benzhydrol (PhCDOHPh) showed a considerable kinetic isotope effect (kH/kD = 5.65 at 298 K). The effect of solvents was studied on the oxidation of 2-propanol (in 19 different organic solvents). The effect of solvents has been investigated after fitting the data in multiparametric equations of Taft's and Swain's. The polar and steric effects of the substituent are also analyzed. A hydride ion transfer mechanism is proposed for this reaction, which proceeds through chromate ester formation.

Selective Oxidation of Lauryl Alcohol to Lauraldehyde under Liquid-Liquid Phase Transfer Catalysis (LL-PTC) with Potassium Chromate as the Oxidizing Agent

In the present research, the selective oxidation of lauryl alcohol to lauraldehyde was studied under liquid-liquid phase transfer conditions using potassium chromate (K2CrO4) as an oxidizing agent and tetrabutyl ammonium bromide (TBAB) as the phase transfer catalyst. Chromium (VI) reagents are used in these oxidations since the alcohols are selectively oxidized to the aldehydes without any over-oxidation. In non-aqueous solutions the oxidation by chromium (VI) does not go to completion as the intermediate partially oxidized material containing chromium must be hydrolysed for oxidation to continue. Liquid-Liquid Phase Transfer Catalysis (LL-PTC) involves the transfer of one reactant from the aqueous phase to the organic phase with the help of small quantities of a phase transfer catalyst which is usually quaternary ammonium or phosphonium salt. It was observed that no over-oxidation products such as lauric acid and high molecular weight esters of lauryl alcohol were formed under the reaction conditions used. The effect of various reaction parameters such as the speed of agitation, temperature, the concentration of lauryl alcohol, the concentration of potassium chromateand phase transfer catalyst loading were studied. A reaction mechanism involving the formation of chromate ester in the organic phase has been proposed and a kinetic model has been developed based on the experimental results obtained.

Revisiting the mechanism of hexavalent chromium ion reduction: The parallel photodecomposition and photocatalytic reduction of chromate(VI) ester

The formation and homogeneous photodecomposition of chromate(VI) ester are often overlooked during the studies of photocatalytic reduction of hexavalent chromium, Cr(VI), in the presence of sacrificial electron donors such as methanol. When photoexcited (λ≤525nm), the chromate(VI) ester, formed by the spontaneous reaction of aqueous Cr(VI) with methanol, undergoes homogeneous stepwise decomposition to trivalent chromium, Cr(III). The addition of semiconductor photocatalysts, i.e., TiO2 and WO3, at relatively low concentrations (<0.5gL−1) was detrimental to the overall rate of Cr(VI) reduction. This was traced to the occurrence of interfacial electron injection from chromate ester to the photocatalyst conduction band, that in turn interrupted inner sphere electron transfer, i.e., photodecomposition. Since it is an interfacial process, the extent of suppression of Cr(VI) reduction is more significant for highly adsorbing TiO2 than on weakly adsorbing WO3. At the same time, even though chromate ester adsorbs strongly on Al2O3, the interfacial electron injection to the insulator particle was unfavorable, and hence no suppression in Cr(VI) reduction was observed. When the TiO2 photocatalyst concentration was increased to 0.5gL−1 and beyond, the photocatalytic reduction path became dominant. Further increase in the photocatalyst concentration saw the linear dependency in the apparent rate of Cr(VI) reduction, as expected for reaction in the non-mass diffusion and non-light scattering limited region. As followed by the electron paramagnetic spectroscopy, both the photodecomposition and photocatalytic reactions follow the stepwise reduction from Cr(VI) to Cr(V) and eventually to Cr(III). For non-photoexcited TiO2 (λ≥420nm), suppression in the rate of Cr(VI) reduction was observed at all concentrations tested. Importantly, the work showcases the parallel but non-independent effects of homogeneous photodecomposition of chromate(VI) ester and its photocatalytic reduction over semiconductor photocatalysts.

KINETICS AND MECHANISM OF OXIDATION OF SECONDARY ALCOHOLS BY MORPHOLINIUM FLUOROCHROMATE

Oxidation of several secondary alcohols by morpholinium fluorochromate (MFC) in DMSO leads to the formation of corresponding ketones. The reaction is first order each in MFC and acidity. The reaction exhibited Michaelis-Menten kinetics with respect to alcohols. The oxidation of 2-propanol- d 2 (MeCDOHMe) exhibited a substantial primary kinetic isotope effect ( k H / k D = 5.16 at 298 K). The oxidation of 2-propanol has been studied in nineteen different organic solvents. The solvent effect has been analysed using Taft's and Swain's multiparametric equations. The reaction was subject to both polar and steric effects of the substituents. A mechanism involving transfer of hydride ion from alcohol to the oxidant, via a chromate ester, has been proposed.

크롬(VI)-헤테로고리 착물(2,4'-비피리디늄 클로로크로메이트)에 의한 치환 벤질 알코올류의 산화반응에서 속도론과 메카니즘

Cr(VI)-heterocyclic complex (2,4’-bipyridinium chlorochromate) was synthesized by the reaction between heterocyclic compound(2,4’-bipyridine) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using 2,4’-bipyridinium chlorochromate in various solvents showed that the reactivity increased with the increase of the dielectric constant (e), in the order : N,N-dimet-hylformamide (DMF) > acetone > chloroform > cyclohexene. In the presence of DMF solvent with acidic catalyst such as hydrochloric acid (HCl solution), 2,4’-bipyridinium chlorochromate oxidized benzyl alcohol (H) and its derivatives (p-CH3, m-Br, m-NO2). Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. The Hammett reaction constant (ρ) was -0.67 (303 K). The observed experimental data have been rationalize the proton transfer occurred followed the formation of a chromate ester in the rate-determining step.

Studies on kinetics and thermodynamics of oxidation of 3,4,5-trimethoxy benzaldehyde, benzaldehyde and N,N-dimethylamino benzaldehyde by tetraethylammonium bromochromate in dimethyl formamide and acetic acid mixture

The oxidation of 3,4,5-trimethoxy benzaldehyde (TMBA), benzaldehyde (BA) and dimethylamino benzaldehyde (DMABA) in N,N-dimethyl formamide (DMF) by tetraethylammonium bromochromate (TEABC) resulted in the formation of the corresponding acids. The reaction is first order with respect to both TEABC and the aldehydes. The reaction is catalyzed by toluene-p-sulfonic acid (p-TsOH). The hydrogen ion dependence has the form: kobs=a+b [H+]. The reaction has been studied in different percentage of DMF–acetic acid mixture. The effect of dielectric constant of the medium indicates the reaction to be of ion–dipole type. Various thermodynamic parameters for the oxidation have been reported and discussed along with the validity of isokinetic relationship. Reason for high rate in the case of oxidation of N,N-dimethylamino benzaldehyde is also described. A mechanism involving formation of a chromate ester intermediate in the slow step has been proposed.

Cr(VI)-헤테로고리 착물(Cr(VI)-Isoquinoline)를 이용한 치환 벤질 알코올류의 산화반응과 속도론에 관한 연구

Cr(VI)-heterocyclic complex[Cr(VI)-isoquinoline] was synthesized by the reaction between of heterocyclic compound(isoquinoline) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using Cr(VI)-isoquinoline in various solvents showed that the reactivity increased with the increase of the dielectric constant(), in the order : cyclohexene, m-Br, m-). Electron- donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant() was -0.69(308K). The observed experimental data have been ratiolized. The hydride ion transfer causes the prior formation of a chromate ester in the rate-determining step.

Cr(VI)-헤테로고리 착물[Cr(VI)-2-methylpyrazine]를 이용한 치환 벤질 알코올류의 산화반응과 메카니즘에 관한 연구

Cr(VI)-헤테로고리 착물[Cr(VI)-2-methylpyrazine]를 합성하여, 적외선 분광광도법(IR), 유도결합 플라즈마(ICP)등으로 구조를 확인하였고, 여러 가지 용매 하에서 Cr(VI)-2-methylpyrazine를 이용하여 벤질 알코올의 산화반응을 측정한 결과, 유전상수(<TEX>${\varepsilon}$</TEX>) 값이 큰 용매 순서인 시클로헥센 < 클로로포름 < 아세톤 < N,N-디메틸포름아미드 용매 하에서 높은 산화반응성을 보였다. 산 촉매(HCl)를 이용한 DMF 용매 하에서 Cr(VI)-2-methylpyrazine은 벤질 알코올(H)과 그의 유도체들(p-<TEX>$CH_3$</TEX>, m-Br, m-<TEX>$NO_2$</TEX>)을 효과적으로 산화시켰다. 전자받개 그룹들은 반응 속도가 감소한 반면에 전자주개 치환체들은 반응속도를 증가시켰고, Hammett 반응상수(<TEX>${\rho}$</TEX>) 값은 Cr(VI)-2-methylpyrazine= -0.65(308K) 이였다. 본 실험에서 알코올의 산화반응 과정은 먼저 크로메이트 에스테르 형성과정을 거친 후, 속도결정단계에서 수소화 전이가 일어나는 메카니즘임을 알 수 있었다. Cr(VI)-heterocyclic complex[Cr(VI)-2-methylpyrazine] was synthesized by the reaction between of heterocyclic compound(2-methylpyrazine) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using Cr(VI)-2-methylpyrazine in various solvents showed that the reactivity increased with the increase of the dielectric constant(<TEX>${\varepsilon}$</TEX>), in the order : cyclohexene<chloroform<acetone <N,N-dimethylformamide(DMF). In the presence of DMF solvent with acidic catalyst such as hydrochloric acid(HCl solution), Cr(VI)-2-methylpyrazine oxidized benzyl alcohol(H) and its derivatives(p- CH3, m-Br, m-NO2). Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant(<TEX>${\rho</TEX><TEX>}$</TEX>) was Cr(VI)-2-methylpyrazine= -0.65(308K). The observed experimental data have been ratiolized. The hydride ion transfer causes the prior formation of a chromate ester in the rate-determining step.

KINETICS AND MECHANISM OF THE OXIDATION OF SOME a-HYDROXY ACIDS BY QUINOLINIUM CHLOROCHROMATE

Normal 0 The oxidation of glycollic, lactic, malic and a few substituted mandelic acids by quinolinium chlorochromate (QCC) in dimethylsulphoxide (DMSO) leads to the formation of corresponding oxoacids. The reaction is first order each in QCC. Michaelies-Menten type of kinetics is observed with respect to the hydroxy acids. Reaction is failed to induce the polymerisation of acrylonitrile. The oxidation of a ‑deuteriomandelic acid shows the presence of a primary kinetic isotope effect ( k H / k D = 5.75 at 298 K). The reaction does not exhibit the solvent isotope effect The reaction is catalysed by the hydrogen ions. The hydrogen ion dependence has the form: k obs = a + b [H + ]. Oxidation of p-methyl mandelic acid has been studied in 19 different organic solvents. The solvent effect has been analysed by using Kamlet’s and Swain’s multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed.

Spectroscopy and Photochemistry of Sodium Chromate Ester Cluster Ions

In the present study, we investigate the spectroscopy and photochemical behavior of chromate ester cluster ions in vacuo in the visible and near-UV. Chromate ester cluster ions Na(n)[CrO3(OCH3)](n+1)(-) (n = 1, 2) are generated by electrospray ionization of sodium dichromate in water and methanol. Upon irradiation with photon energies between 2 and 5.6 eV, dissociation occurs. The photodissociation spectra of these ions are very similar to the UV/vis absorption spectra of the sample solution with solvatochromic shifts less than 0.1 eV. The electronic excitations in this photon energy range are assigned to ligand-to-metal charge transfer from the oxygen ligands to the chromium within the chromate ester. Fragment channels corresponding to intracluster reactions involving reduction of the metal centers as well as evaporative processes leading to the loss of a neutral Na[CrO3(OCH3)] salt unit are observed. The results are discussed in the framework of organometallic redox mechanisms and density functional theory.

크롬(VI)-4,4'-Bipyridine 착물에 의한 치환 벤질 알코올류의 산화반응 속도론과 메카니즘

Cr(VI)-4,4'-bipyridine complex(4,4'-bipyridinium dichromate) was synthesized by the reaction of 4,4'-bipyridine with chromium trioxide in H2O, and characterized by IR, ICP. The oxidation of benzyl alcohol using 4,4'-bipyridinium dichromate in various solvents showed that the reactivity increased with the increase of the dielectric constant(e), in the order: cyclohexene<chloroform<acetone<N,N'-dimethylformamide. In the presence of hydrochloric acid(HCl solution), 4,4'-bipyridinium dichromate oxidized substituted benzyl alcohols (p-CH3, H, m-Br, m-NO2) smoothly in N,N'-dimethylformamide. Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant(ρ) was -0.63(303K). The observed experimental data have been rationalized as follows; the proton transfer occurs after the prior formation of a chromate ester in the rate determining step.

Oxidation of 3,4,5-trimethoxybenzaldehyde by pyridinium fluorochromate in N,N-dimethyl formamide medium: A kinetic and mechanistic study

Kinetics of oxidation of 3,4,5-trimethoxybenzaldehyde (TMBA) by pyridinium fluorochromate (PFC) in N,N-dimethyl formamide (DMF) medium in the presence of toluene para-sulfonic acid (TsOH) has been studied. The reaction is first order each in [PFC], [TMBA] and [TsOH]. The rate of oxidation remains unaltered by the variation of NaClO4 but addition of MnSO4 decreases the rate. The effects of the dielectric constant of the medium and the ionic strength indicate the reaction to be of ion–dipole type. The stoichiometry of the reaction is 1:1 and the product of oxidation is the corresponding 3,4,5-trimethoxybenzoic acid and Cr(III). The reaction rates have been determined at different temperatures and the activation parameters were calculated. The reaction failed to induce the polymerization of acrylonitrile, i.e., absence of free radicals. A suitable mechanism involving the slow formation of an unstable chromate ester and a rate law has been proposed.

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation of Some Meta- and Para-Substituted Benzyl Alcohols in Non-Aqueous Media

Abstract Tetraethylammonium bromochromate (TEABC) oxidation of 18 meta- and para-substituted benzyl alcohols (BnOH) in dimethylsulphoxide (DMSO) leads to the formation of corresponding aldehydes. The reaction is first order each in TEABC and the alcohols. The reaction is catalyzed by hydrogen ions. The hydrogen ion dependence has the form: k obs=a+b[H+]. The oxidation of αα ´-dideuterio benzyl alcohol exhibited a substantial primary kinetic isotope effect (k H/k D=5.71 at 303 K). Oxidation of benzyl alcohol was studied in 17 different organic solvents. The solvent effect has been analyzed using Kamlet's and Swain's multi parametric equation. A correlation of data with Kamlet–Taft solvatochromic parameters (α, β, π *) suggests that the specific solute-solvent interactions play a major role in governing the reactivity. A mechanism involving a hydride ion transfer via chromate ester is proposed.

Studies on the kinetics of tripropylammonium fluorochromate oxidation of some aromatic alcohols in non-aqueous media

The oxidation of benzyl alcohol (BnOH) and a few para-substituted benzyl alcohols by tripropylammonium fluorochromate (TriPAFC) in dimethylsulphoxide (DMSO) leads to the formation of corresponding aldehydes. The reaction is first order each in TriPAFC and the alcohols. The reaction is catalysed by hydrogen ions. The hydrogen ion dependence has the form: k obs = a + b[H +]. The oxidation of α α′-dideuterio benzyl alcohol exhibited a substantial primary kinetic isotope effect ( k H/ k D = 5.45 at 303 K). Oxidation of benzyl alcohol was studied in 19 different organic solvents. The solvent effect has been analysed using Kamlet's and Swain's multiparametric equation. A mechanism involving a hydride ion transfer via chromate ester is proposed.

Oxidation of some α-hydroxy acids by tetraethylammonium chlorochromate: A kinetic and mechanistic study

The oxidation of glycolic, lactic, malic, and a few substituted mandelic acids by tetraethylammonium chlorochromate (TEACC) in dimethylsulfoxide leads to the formation of corresponding oxoacids. The reaction is first order each in TEACC and hydroxy acids. Reaction is failed to induce the polymerization of acrylonitrile. The oxidation of α-deuteriomandelic acid shows the presence of a primary kinetic isotope effect (kH/kD = 5.63 at 298 K). The reaction does not exhibit the solvent isotope effect. The reaction is catalyzed by the hydrogen ions. The hydrogen ion dependence has the following form: kobs = a + b[H+]. Oxidation of p-methylmandelic acid has been studied in 19 different organic solvents. The solvent effect has been analyzed by using Kamlet's and Swain's multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 50–55, 2010

Influence of cerium(IV) and manganese(II) on the oxidation of D-galactose by chromium(VI) in the presence of HClO4

The kinetics and mechanism of the oxidation of D-galactose by chromium(VI) in the absence and presence of cerium(IV) and manganese(II) were studied spectrophotometrically in aqueous perchloric acid media. The reaction is first order in both [D-galactose] and [H+]. The cerium(IV) inhibits the oxidation path, whereas manganese(II) catalyzes the reactions. The observed inhibitory role of cerium(IV) suggests the formation of chromium(IV) as an intermediate. In the manganese(II) catalyzed path, the D-galactose-manganese(II) complex was considered to be an active oxidant. In this path, the complex forms a ternary chromate ester with chromium(IV) which subsequently undergoes acid catalyzed redox decomposition (one-step three-electron transfer: Indian J. Chem., 2004, vol. 42A, p. 1060; Colloids and Surfaces, 2001, vol. 193, p. 1) in the rate determining step. On the basis of kinetic data, the mechanism of D-galactose oxidation is proposed for parent, the manganese(II) catalyzed and cerium(IV) — inhibited reactions. The activation parameters Ea = 59 kJ ΔH# = 57 kJ mol−1, and ΔS# = −119 J K−1 mol−1 are calculated and discussed. Reaction products are also examined.

Kinetics and Mechanism of the Oxidation of Some α-Hydroxyacids by Morpholinium Chlorochromate

The oxidation of glycollic, lactic, malic and a few substituted mandelic acids by morpholinium chlorochromate (MCC) in dimethylsulfoxide (DMSO) leads to the corresponding oxoacids. The reaction is first order each in MCC and hydroxyacid. The reaction failed to induce the polymerisation of acrylonitrile. The oxidation of α–deuteriomandelic acid shows a primary kinetic isotope effect ( kH/ kD= 5.63 at 298 K) but does not exhibit a solvent isotope effect. The reaction is catalysed by hydrogen ions according to: kobs= a + b[H+]. The oxidation of p-methyl mandelic acid has been studied in 19 different organic solvents and the solvent effect analysed using Kamlet's and Swain's multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed.

Kinetics and mechanism of the oxidative cleavage of ketones by quinolinium dichromate

The kinetics of the oxidation of ketones (cyclic and arylalkyl) by quinolinium dichromate (QDC), in an acid medium, has been investigated. The rate of the reaction was proportional to the concentrations of the ketone, QDC, and acid. The order of reactivity for the cyclic ketones was C6 > C8 > C5 > C7 (ring size) and acetophenone > propiophenone > butyrophenone > valerophenone (arylalkyl ketones). A kinetic isotope effect, k H/k D = 5.80, was observed (cyclohexanone and cyclohexanone-d4). The rate data have been rationalized using conformational considerations. The mechanism involved the attack of the protonated QDC on the enol form (of the ketone), in the rate-determining step, forming a cyclic chromate ester. Subsequent cleavage of the carbon-carbon bond of the ester yielded carboxylic acids as the products.